feat(input): #83 Input Manager + auto-commit the device layer in the frame loop

Fixes the papercut: the generated frame loop called rt_poll() (feeding only
Input.key) but never input_poll(), so Input.active/key_down/mouse/pad read
empty unless the game called Input.poll() by hand. The loop now calls
input_poll() when the game uses any Input runtime method — committing the
held-key/mouse/gamepad state, and record/replay — and stores its return as the
frame key so Input.key still works. A game using no Input runtime keeps the
plain rt_poll path, byte-identical.

Adds the Input-Manager API: Input.action(name,key) ships a default binding
(kept if already bound, so a rebind/loaded map isn't clobbered),
Input.bind_pad(name,button) makes an action device-agnostic (keyboard OR pad),
and Input.active/just_pressed/just_released read the multi-key device layer
with clean on-press/on-release edges (deterministic, dispatch-free — a handler
polls the edge; a replay fires identically).

Examples input_manager + input_auto, 5 docs pages. Full suite 107/0, goldens
byte-identical, fixpoint holds.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/language/input/input-action.md

@@ -0,0 +1,23 @@
+---
+id: input-action
+name: Input.action
+category: input
+kind: namespace-method
+tokens: Input.action
+sig: Input.action(action: str, key: int) -> void
+tip: Register a default key binding for an action (kept if already bound).
+order: 31
+ns: Input
+member: action
+---
+
+Registers a <em>default</em> keyboard binding: it binds <code>key</code> to the action only if the action has no key bound yet. Ship your defaults with <code>Input.action</code> in a Boot handler; a player's later <a href="input-rebind.html"><code>Input.rebind</code></a> (or a loaded key-map) is not clobbered, and re-running the defaults is idempotent. Combine with <a href="input-bind_pad.html"><code>Input.bind_pad</code></a> for a device-agnostic action, and read it with <a href="input-active.html"><code>Input.active</code></a>.
+
+```ludic
+program Defaults {
+  entry {
+    Input.action("jump", ' ')
+    if Input.active("jump") { print(1) }
+  }
+}
+```